As an update to your question since it's been a few months, Multiple Answer questions support partial credit. If you're interested in that quiz type, see How do I create a Multiple Answer question in New Quizzes?
Agreed. It should be easy to do partial credit on ordering. It already has the individual items marked as correct or incorrect...Much easier than partial credit for fill-in-the-blanks where it is possible to have a large number of correct answers, particularly if you don't take off for spelling errors...
I believe there is confusion because there is a question option titled "Multiple Answer"
In truth all questions with multiple answers (categorization, ordering, matching, and multiple answer) must have the option for partial credit - not just the question type named "Multiple Answer."
Someone from Canvas, please acknowledge this.